This invention relates to the continuous monitoring of an atmosphere to detect the presence of combustible gas or vapour, for example the presence of methane in coal mines.
Methane is the major flammable component of fire damp in coal mines and for reasons of safety it is necessary to measure quantitatively the presence of methane in coal mines, to record the concentration of methane and/or to set off an alarm when the concentration reaches a predetermined level. Methane concentration has been measured by means of detectors which include a catalyst for the oxidation of methane. Methane and air react on the surface of the catalyst. The heat evolved from this reaction is measured and can be used to provide a measure of the amount of methane present. Such detectors normally require a transducer which performs three functions: (1) it acts as a catalyst for the oxidation of methane; (2) it acts as a heater to raise the catalyst to the temperature at which it will catalyse the oxidation reaction; and (3) it acts as means for sensing the heat evolved in the oxidation reaction. In some detectors these three functions are performed by the same item, for example a coil of platinum wire which is part of a resistance bridge and through which a current is normally passed. The passage of the current heats the coil of platinum wire to a temperature at which it will catalyse the reaction between methane and oxygen. The heat evolved during the reaction further heats the coil of platinum wire and thereby increases its resistance, which is detected by means of the resistance bridge. Such detectors are described in an article in "The Mining Engineer", January 1969, pgs. 237 to 244.
Methane concentration can also be measured by means of a semiconductor transducer whose resistance varies with the concentration of flammable gas. Such semiconductor transducers and their use in detectors for flammable gas are described, for example, in U.S. Pat. Nos. 3,603,954 and 3,631,436.
A problem which arises when using such detectors is that they are rendered inoperative if the transducer becomes inactive, for instance in the case of a catalytic transducer as a consequence of poisoning or inhibition. There are usually present in a mine substances which, if they come into contact with the catalyst, will reduce the efficiency of the catalyst or render it useless. A detector containing an inoperative transducer, for example a poisoned catalyst, will give a reading indicating the presence of no methane regardless of whether methane is present or not and it is not possible to tell without carrying out tests on the detector whether it is malfunctioning or not.